In the modern era, the fleet flagship has usually been an aircraft carrier, but that changed in 2010 with the assignment of HMS Albion.
[2] Finally, the flag reverted to an aircraft carrier when HMS Queen Elizabeth took up her position in January 2021.
Technically, the fleet flagship would be the ship that would host the two-star maritime battlestaff headquarters (such as COMUKMARFOR, a Rear Admiral who normally is based ashore) for operations.
Generally, a flagship is a ship in which an Admiral (or a Commodore) flies his flag (or broad pennant).
Since then the Fleet Commander has flown his flag from these headquarters at the shore establishment HMS Excellent.